February 7, 2012

A Bronze Matryoshka Doll: The Metal In The Metal In The Metal

by editor

New way to highly efficient catalysts and nanotubes with unusual symmetry

A doll in a doll, and then one more, enveloping them from the outside — this is how Thomas Faessler explains his molecule. He packs one atom in a cage within an atom framework. With their large surfaces these structures can serve as highly efficient catalysts. Just like in the Russian wooden toy, a hull of twelve copper atoms encases a single tin atom. This hull is, in turn, enveloped by 20 further tin atoms. Professor Faessler's work group at the Institute of Inorganic Chemistry at the Technische Universitaet Muenchen (TUM) was the first to generate these spatial structures built up in three layers as isolated metal clusters in bronze alloys.

Particularly fascinating are the images the researchers use to explain these chemical compounds and their properties. In the laboratory the substance is an unimpressive, fine, grayish-black powder, yet the structure models are in color and in various nested shapes. These powders, with their large surfaces, are interesting as an interim step for catalysts that transfer hydrogen, for instance. Similar structures made of silicon could be used in solar cells to capture light from the sun more effectively.

Most people view metals as uniform materials with a rather unspectacular structure. The metal compounds from Faessler's institute are quite the opposite. His desk is piled high with various multicolored cage models with yellow spheres representing copper atoms and blue ones for tin. The analogy to the carbon spheres that caused a sensation as Buckyballs can not be overlooked. Here, too, there are geometric structures made up of triangles, pentagons and hexagons. However, they are not made of carbon: heavier metals such as tin and lead can also form such isolated cage structures.

"We are basically interested in alloy structures that are out of the ordinary," says Faessler. Bronze, for example: this mixture of copper and tin, which was discovered early on and lent its name to an entire age of humanity, has a crystalline structure; the atoms of the two components are distributed evenly throughout the entire crystal and are densely packed together.

The new bronzes from the Faessler laboratory are different. The PhD candidate Saskia Stegmaier melted a particularly pure form of copper wire and tin granulate under special conditions — protected from air and moisture in an argon atmosphere. The bronze produced in this manner was then sealed into an alkali metal such as potassium in an ampoule made of tantalum. The melting point of tantalum is 3,000 degrees Celsius, making it particularly well suited as a vessel for binging other metals into contact with each other.